Measurement in Ultrasonography: Does It Matter?

Organ size estimation using ultrasound is subjective. Interpretation of the ultrasound examination based on size alone is not advisable. When there is suspected change in the size of a certain organ, it should be interpreted with other signs of disease such as change of echogenicity and presence of clinical signs. The important practical aspect of size measurement in ultrasound is for future comparison in follow up examinations. This could be useful to determine whether there is a progression of disease or response to treatment. There is no reliable way to measure the size of the liver. Rounding of the margins and excessive caudal extension of the liver suggest hepatomegaly. A small liver is suspected when there is poor visualization of the liver. The size of the spleen in dogs and cats varies depending on many factors. Sonographic determination of splenomegaly is often subjective. A more reliable way to measure the size of spleen is through radiography.

In cats, the spleen is always small and an enlargement of the spleen is easier to recognize. Although a positive correlation of kidney length and volume with body weight has been reported, it is not widely used clinically due to the wide range of standard deviation. However, linear kidney measurement in cats is more reliable as there is less variation of the body size. It is more difficult to determine mild enlargement of the kidneys when there is no alteration of the echogenicity and echotexture. Slightly smaller kidney size is commonly seen in older dogs and cats due to chronic renal disease. A mild increased size of the renal pelvis has been reported to be normal, secondary to intravenous fluid administration, or associated with disease condition such as pyelonephritis and ureter obstruction. Marked dilatation of the renal pelvis (hydronephrosis) is due to obstruction of the ureter. The width of the pancreas in dogs and cats is 1–3 cm and 1 cm respectively. A detectable pancreatic change of echogenicity or contour is usually accompanied with an increase in the size of the pancreas. Unfortunately, size change does not always mean that there is pancreatic pathology. The canine prostate size is variable. Its size increases with age and is small in neutered males. Although there are studies indicating prostate size estimation using ultrasound is reliable, it is still very subjective as differentiating between normal and diseased prostate is difficult.

The volume of the gallbladder and urinary bladder varies in both dogs and cats. Some of them appear large but have no clinical significance. The wall of the gallbladder should appear as a thin echogenic line in normal dogs and cats. Thickening and the presence of a measurable gallbladder wall most likely indicates edema or inflammation of the wall. The diameter of the common bile duct in dogs and cats is <3 mm and <4 mm respectively. Dilation of the common bile duct most likely indicates obstruction. The urinary bladder wall should be <2.3 mm for dogs and <1.7 mm for cats. Increased urinary bladder wall thickness should be interpreted with other changes and history. Thickness of the gastrointestinal tract in normal dogs and cats has been reported. Generally, the gastric thickness of dogs and cats should be less than 5 mm and 3.6 mm, respectively. The thickness of the small intestines in cats is more consistent and mostly <2.5 mm. As for dogs, the thickness of the small intestines varies depending on the body size. However, it should be less than 6 mm for the duodenum and 5 mm for jejunum. The thickness of the wall of the colon is 2–3 mm in dogs and 1.4–2.5 mm in cats. Increased wall thickness without disruption of the five layer appearance of the gastrointestinal tract is usually due to inflammatory bowel disease except when there is increased thickness of the muscularis layer, which may indicate lymphosarcoma. A normal adrenal gland size has been reported in dogs. The maximal diameter of the adrenal gland has been found to be the most reliable indicator of its size. As a general rule, a diameter of 7.4 mm has been suggested to be used as the upper limits of normal. Unfortunately, overlapping in size between normal dogs and those with adrenal gland abnormality exist. Approximately 25% of dogs with pituitary dependent hyperadrenocorticism (PDH) do not have adrenal gland enlargement, and about 20% of dogs without any evidence of adrenal diseases had adrenal size more than the proposed upper limits of 7.4 mm.

Enlargement of the canine adrenal glands is suggestive of PDH, neoplasia or nodular hyperplasia. In many instances, abnormalities of the adrenal glands lead to an increase in size alone without any nodules or masses. In cats, a study using 10 normal adult cats showed that the normal size to be 4.3±0.3 mm.

As a general rule, size measured should be interpreted with care, and should always correlate with clinical signs.